This invention relates to an image processing apparatus equipped with a main CPU mainly for controlling electric circuitry, and a subordinate CPU mainly for controlling other systems, such as a recording/reading mechanism.
In such a conventional apparatus of this kind as a facsimile apparatus, primarily the following methods are used as measures to prevent "runaway" (a phenomenon in which a CPU goes out of control, as when the CPU clock develops an abnormality, thus causing the program to jump to the wrong location) of the subordinate CPU which controls such systems as the recording system of the facsimile apparatus:
(1) The subordinate CPU is monitored by the main CPU. When the main CPU senses an abnormality in the subordinate CPU, either the subordinate CPU or a high-energy power supply for operating the recording system, etc., controlled by the subordinate CPU is shut down under the control of the main CPU.
(2) The system clock of the subordinate CPU is monitored by a watching circuit or the like, and a high-energy power supply is shut down when the system clock stops.
However, a major problem is encountered in the conventional method (1) described above. Specifically, if the main CPU should develop runaway, it will no longer be able to monitor the subordinate CPU. When both the main CPU and subordinate CPU develop runaway, the recording system can malfunction, the motor and thermal head may emit smoke and burn, and develop into a fire.
In this case, an expedient can be adopted in which the main CPU is monitored by a watching circuit and the power supply of the recording system is turned off when the system clock of the main CPU stops.
With the conventional watching circuit, however, any instantaneous abnormality in the system clock cannot be sensed and runaway cannot be detected without fail. Moreover, since the reaction speed of a watching circuit is slow, there are instances where the power supply of the recording system is turned off after the main or subordinate CPU has developed runaway and the recording system components such as the motor or thermal head have already begun to smoke and burn. Thus, this expedient does not fully perform its function in terms of preventing emission of smoke and burning.
Likewise, the conventional method (2) described above also does not completely play its role in terms of the reaction speed of the watching circuit and the perfection with which the watching circuit monitors runaway of the subordinate CPU.